Description:

The invention pertains to a self-healing material.

Self-healing materials are materials that autonomously repair a damage caused by (usually) an external force and are well known in the prior art. The two mostly investigated strategies are the inclusion of reactive components that are released upon damage, and react to repair the properties of the material and the incorporation of reversible bonds, such as those based on multiple hydrogen bonds.

Document US 2004/0114933 A1 discloses a self-healing cross-linked polymer, which is made by polymerising furan monomers and maleimide monomers via Diels-Alder reaction.

Document US 2008/0173382 A1 describes a self-healing mechanism based on encapsulated sulfur.

In document CN 101153108 A also a self-healing material is disclosed, which is based on an encapsulation mechanism. The microcapsules and encapsulation have as a main limitation that the healing process just can take place once at the same place.

Aim of the present invention is a self-healing material, which healing effect can easily be accomplished and occurs in the entire material without a local restriction inside the material. This means that there is no location in the material where the healing effect is absent.

It is another object of the present invention to overcome or at least to minimize the limitations of the prior art. The objects mentioned above are achieved by a self- healing polymer, wherein the polymer comprises at least two healing moieties, whereby each of the at least two healing-moieties comprises at least one disulfide group (-S-S-).

Generally, the polymers that are suitable for the invention are obtainable from one or more monomers by measures known per se, such as (but not restricted to) polymerisation, polycondensation, polyaddition reactions, whereby each of the monomers has an average functionality of at least 2.0. Polymers, obtained from monomers which have an average functionality of 2.0 are commonly referred to as linear polymers.

Due to the use of at least two disulfide-groups in the self-healing polymer the self- healing effect can take place in the entire polymer. Moreover, self-healing can be repeated several times and in addition to this already at room temperature - like 20 ⁰ C.

In case of a linear polymer obtained - as mentioned above - from monomers with an average functionality of 2.0), the presence of self-healing moieties exhibit an advantage compared to the classical forms of healing, such as re-melting of the polymer. First of all, re-melting may destroy or weaken the structural integrity of the polymeric structure. Secondly, in case of linear polymers that do not possess a melting point (for example do not melt without decomposition) the presence of the healing moieties could be the only possibility for healing. Examples to this end may be the high-crystalline linear polymers, such as polyparaphenylene terephthalamide. Preferably, the self-healing polymer is a crosslinked polymer.

A crosslinked polymer is obtained from one or more monomers, whereby each of the monomers having an average functionality of greater that 2.0. It is especially preferred if at least one of the monomers has an average functionality of 3.0, more preferred of 4.0.

It is preferred that self-healing is achieved by interchange reaction via the disulfide-groups. Due to the interchange reaction a self-healing effect depends only on the distribution of the disulfide-groups in the polymer. A homogeneous distribution enables also a homogeneous self-healing effect over the total polymer. It is preferred that the concentration of the disulfide groups is above 1 wt%. More preferably the concentration of the disulfide groups is above 11 wt% and most preferred is a concentration of the disulfide groups of higher than 19 wt%.

Preferably, the interchange reaction is reversible. Such reversible interchange reaction is conveniently achieved by the capability of the disulfide-groups to undergo an interchange reaction by cleavage of the chemical bonding between the two sulfides in a disulfide-group and subsequent reformation of the chemical bonding to different sulfides also formed by cleavage of the chemical bonding between two sulfides of other disulfide groups within the polymer. The interchange reaction between the various disulfide groups within the self-healing polymer occurs in a temperature depending equilibrium.

It is preferred that the mechanical properties of the self-healing polymer after self- healing are retained to at least 60% of the original mechanical properties present before having applied the mechanical stress, such as a break. More preferably the mechanical properties of the self-healing polymer after self-healing are retained to at least 80% of the original mechanical properties. It is most preferred that the mechanical properties of the self-healing polymer after self-healing are retained to at least 100% of the original mechanical properties. Due to the ability of the disulfide groups re-connect with the same or with another sulfide partners; the polymer is able to heal damages without the need of additional means like glue, heat or pressure. In case of a crosslinked polymer the network of the crosslinked polymer can be destroyed partially or completely by a damage, whereby the disulfide groups build up a "new" network. Due to the healing effect therefore the topology of the network is changed, but not the chemical structure of the polymer. Hence, the connectivity of crosslink points is altered. The crosslinked polymer in the "new" network has preferably the same mechanical properties as the crosslinked polymer in the "old" network. It should be pointed out that the disulfide groups in the polymer could possibly cleave and reconnect within the polymeric system all the time (and not only in case of damage).

Advantageously the polymer is able to heal a cut or damage with near 100% recovery of modulus, elongation at break and/or tensile strength, which are generally sum up as mechanical properties.

Preferably, the crosslinked polymer containing the self-healing moieties (or healing moieties) is a cured resin. In the course of this invention with the tern "resin" materials are meant that are capable of curing (polymerizing and crosslinking) when mixed with a catalyzing agent or hardener. Resins are typically viscous liquids manufactured by estehfication or soaping of organic compounds. The classic variety is epoxy resin, manufactured through polymerization-polyaddition or polycondensation reactions, used as a thermoset polymer for adhesives and composites. Most common epoxy resins are produced from a reaction between epichlorohydrin and bisphenol-A. Other examples for resins are polyurethane resins or unsaturated polyester resins.

Thus, it is especially preferred, if the crosslinked polymer is a cured epoxy resin. The epoxy resin can be cross-linked with thiols in a base-catalyzed addition reaction. The epoxy resin containing disulfide-groups in its structure is e.g. cross- linked with a tetra functional thiol in the presence of 1 wt% of 4- dimethylaminopyridine at 60 ⁰ C or lower. This process takes about 80 minutes. The resulting crosslinked polymer is preferably a transparent thermoset rubber, with a glass transition temperature of approximately -35 ⁰ C.

Another object of the present invention is a process for manufacturing a self- healing polymer. The process is characterized in that during a polymerisation reaction at least two healing moieties are introduced in the polymer, whereby each of the at least two healing moieties comprises at least one disulfide group.

In one embodiment it is preferred that the process comprises the step of copolymerisation of at least two monomers from which at least one of the monomer is trifunctional and whereby at least one of the monomers contains disulfide groups. One example of this process is the copolymerisation of a stoichiometric mixture of an epoxy resin containing disulfide groups in its structure with a trifunctional thiol as a crosslinking agent. Another example is the copolymerization of a stoichiometric mixture of an isocyanate with a trifunctional thiol containing disulfide groups in its structure.

In one other embodiment it is preferred that the process comprises the step of copolymerisation of at least two monomers from which at least one monomer is trifunctional and wherein both monomers contain disulfide groups. One example of this process is the copolymerization of an stoichiometric mixture of an epoxy resin containing disulfide groups in its structure with a difunctional primary amine with disulfide groups in its structure, as for example 2-[(2- aminoethyl)disulfanyl]ethylamine. Another example could be the copolymerization of allyl monomers containing disulfide groups in its structure, as for example 3- (allyldisulfanyl)-i -propene, with a trifunctional thiol with disulfide groups in its structure.

The self-healing polymer is preferably used for coatings, asphalts, biomedical applications, automobile industry and aerospace industry. In the field of coating the self-healing polymer can be used as isolation material or surface protection. Biomedical applications for examples are support devices for bones and vascular or implants for extremities. In automobile and aerospace industry the self-healing polymer can be used for car or aircraft body panels or for interior devices - for examples dashboards or trays.

The self-healing polymers have a glass transition temperature of preferably lower than 30 ⁰ C, even more preferred of lower than 0 ⁰ C.

The present invention is further described in the following, with reference to the accompanying drawings and examples

Brief description of the drawings

Figure 1 shows one preferred reaction equation for producing a self- healing crosslinked polymer based on epoxy resin.

Figure 2 shows a FTIR/ATR spectra of an stoichiometric mixture of

EPS25 with tetrathiol before and after curing at 60 ⁰ C.

Figure 3 shows a Raman spectra of an stoichiometric mixture of EPS25 with tetrathiol before and after curing at 60 ⁰ C.

Figure 4 shows a DMA plot of a cured EPS25 with tetrathiol.

Figure 5 shows the compliance as a function of time for the self-healing crosslinked polymer (obtained from EPS25 and tetrathiol) and the control material.

Figure 6 shows a DMA plot of a control material.

Figure 7 shows schematically the experiment that was done to quantify the self-healing properties.

Figure 8 A to C show the stress-strain curves of different samples (EPS25 cured with tetrathiol). (A) virgin sample, (B) at different times of healing at 60°C, (C) at multiple healing times of one sample healed at 60 ⁰ C for 1 hour. Figure 9 A and B show the optical images of the self-healing crosslinked epoxy resin (EPS25 with tetrathiol) Figure 10 shows schematically the recovery of strength in function of the concentration of disulfide links in a cross-linked epoxy resin. Figure 11 shows another preferred reaction equation for producing a self-healing crosslinked polymer based on polythiocarbamate. Figure 12 A to C show schematically an evaluation of the self-healing properties from a polythiocarbamate.

Manufacturing of a self-healing crosslinked polymer according to the invention:

Example 1 :

In a preferred embodiment a self-healing crosslinked polymer was obtained by curing of epoxydized polysulphide EPS25 (Akzo-Nobel, epoxy equivalent = 930 g/eq) with pentaerythritol tetrakis(3-mercaptopropionate) (Aldrich, hereinafter referred to as Tetrathiol) as a curing agent in the presence of 1 wt % of 4- dimethylaminopyridine (Aldrich, hereinafter referred to as DMAP) at 60 ⁰ C for 2 hours (see figure 1 ). The molar ratio used in the copolymerization of EPS25 and Tetrathiol was 2:1 respectively. The sample was prepared by mixing and manual stirring in a mortar the corresponding amounts of EPS25, tetrathiol and DMAP. Then, it was transferred in a mould and cured in an oven at 60 ⁰ C for 2 hours.

Figure 2 shows the FTIR/ATR spectra before 4 and after 5 curing at 60 ⁰ C and in figure 3 is the Raman spectrum before 4 and after 5 curing at 60 ⁰ C shown. The FTIR and Raman spectrum of the cured sample showed that the absorption bands of the functional groups (epoxy and thiol groups) disappeared totally indicating that the resin was completely cured. Figure 2 shows the FTIR spectra of a stoichiometric mixture of EPS25 and Tetrathiol before 4 and after 5 the curing process. In the FTIR-spectrum of the unreacted sample, a broad absorption band between 860 and 815 cm ^"1 , which can be assigned to a stretching vibration of the oxirane ring, is observed together with a weak absorption band at 2536 cm ^"1 , which can be assigned to the S-H stretching vibration of the thiol group. The complete disappearance of these two absorption bands indicates that the conversion of the cross-linking reaction is close to 100% and that the monomers are incorporated into the network.

Due to the low intensity of the thiol absorption band in FTIR spectroscopy, Raman experiments were performed to confirm the complete disappearance of this band (see figure 3).

The resulting material is a transparent thermoset rubber, with a glass transition temperature of approximately -35 ⁰ C. For conventional cross-linked polymers the rubbery plateau modulus extends to the temperature at which the material decomposes [W. D. Callister, Materials Science and Engineering. An Introduction (Wiley, New York, ed. 7, 2007), p. 487.]. Figure 4 shows (partially) in semi-logarithmic form the temperature dependence of storage modulus E' (curve 1 ), loss modulus E" (curve 2) and tangent delta (curve 3) for the epoxy with disulfide groups in its structure. However, the obtained cross-linked rubber (see figure 4) shows an unusual ability to flow at temperatures above 100 ⁰ C. This indicates that the disulfide links disconnect and the chain segments gain enough mobility to flow. This flow capability is required to have a self-healing property.

To study the flow behavior at different temperatures, creep experiments were performed using a rheometer at constant low shear stress (100 Pa). In figure 5 the compliance as a function of time is shown at 60, 80 and 100 ⁰ C. Disulfide group dissociation in the network is higher with temperature than for the increase of the creep compliance. Curve 6 shows an epoxy resin with disulfide groups according to the invention at 100 ⁰ C. Curve 7 shows an epoxy resin with disulfide groups according to the invention at 80 ⁰ C and curve 8 shows an epoxy resin with disulfide groups according to the invention at 60°C. In curve 9 a control material (epoxy resin without a disulfide group) at temperatures of 60 ⁰ C, 80°C and 100°C is shown. Example 2:

In another preferred embodiment the self-healing crosslinked polymer with disulfide group is obtained by curing of 4,4'-Methylenebis(phenyl isocyanate)

(Aldrich, hereinafter referred to as MDI) with a trifunctional thiol, Thioplast G21

(Akzo-Nobel, M _n = 2267 g/mol) (see figure 11 ). The molar ratio used in the copolymehzation of MDI and Thioplast G21 was 3:2 respectively.

The sample was prepared by mixing and manual stirring in a mortar the corresponding amounts of MDI and Thioplast G21. Then, it was transferred in a preheated mould, degassed by vacuum and cured under argon atmosphere for 2 hours at 80 ⁰ C and overnight at 120 ⁰ C. The resulting polythiocarbamate has a glass transition temperature of approximately -17 ⁰ C.

Control material:

To confirm that the flow behavior is only due to the disconnection of disulfide groups and not due to physical effects, a control experiment was performed with a material that has a similar cross-link density, but without disulfide groups in its structure. As depicted in figure 5, the control material does not show a critical stress to induce flow. Furthermore, the storage modulus (see figure 6) shows a rubbery plateau, which is typical for cross-linked polymers. Figure 6 shows (partially) in semi-logarithmic form the temperature dependence of the storage modulus E' (curve 1 ) and the temperature dependence of the loss modulus E" (curve 2) and tangent delta (curve 10) for the control material. The reference material (control material) is obtained by a copolymerization of tetrathiol with poly(ethylene glycol-b/oc/c-propylene glycol-b/oc/c-ethylene glycol) end-capped with glycidyl moieties. The glycidyl end-capped thblock copolymer was selected for the synthesis of the reference material, since it yields an amorphous cross-linked polymer. The use of glycidyl end-capped poly(ethylene glycol) would have led to a semi-crystalline cross-linked polymer with quite different properties.

Used instruments: FTIR spectra were obtained with a FTIR spectrophotometer Excalibur 3000 from BioRad with a resolution of 4 cm ^"1 in the absorbance mode. An FTIR Spectra was obtained by placing the material on the diamond crystal of a Specac Golden Gate attenuated total reflection (ATR) setup placed in a BioRad Excalibur 3000 spectrometer with a resolution of 4 cm ^"1 in the absorbance mode. Raman spectroscopy of the initial monomer mixture and the cured epoxy resin was performed using a RamanStation spectrometer from Avalon Instruments with a laser with an excitation line of 784 nm.

The rubber plateau modulus was obtained by dynamic mechanical thermal analysis (DMTA), where the sample is sinusoidally deformed during a temperature ramp. The sample for DMTA had approximate dimensions of 6 mm long x 4 mm wide x 2 mm thick. The sample was mounted in a film tension clamp in a TA Instruments Q800 from -100 to 140 ⁰ C at a heating rate of 3 °C/min. The DMTA were strain-controlled with constant amplitude of 10 μm. A preload force of 0.01 N and force track factor of 110% was used.

The dynamic shear measurements were performed on a strain-controlled AR-G2 rheometer (TA Instruments) by using an 8 mm parallel-plate geometry and disk- shaped specimens (8 mm diameter; 1.5 mm thick).

Glycidyl end-capped poly(ethylene glycol-b/oc/c-propylene glycol-b/oc/c-ethylene glycol) was prepared with some adaptations according to the literature [X-P. Gu, I. Ikeda, M. Okahara, Synthesis. 6, 649, (1985)]. Briefly, a mixture of 138.5 g (1.5 mol) epichlorohydrin, 84 g (1.5 mol) potassium hydroxide and 1 g (2.94 mmol) tetrabutylammonium hydrogen sulfate, was prepared to which 50 g (26.3 mmol) of PEG-PPG-PEG (Aldrich, M _n ~ 1900) was slowly added under vigorous stirring in an ice bath. Subsequently, the mixture was stirred for 18 hours at room temperature. The reaction mixture was filtered and evaporated to dryness. The obtained product was purified by precipitation of the dissolved polymer in 40 milliliter of chloroform into pentane, yielding 39 g of glycidyl end-capped PEG- PPG-PEG (epoxy equiv. = 900 g/equiv.). The epoxy equivalent was determined by the Jay-Dijkstra-Dahmen method, variation Ciba. [R. Dobinson, W. Hofmann, B. P. Stark. The Determination of Epoxide Groups (Pergamon Press Ltd., London, ed. 1 , 1969), p. 40.].

Evaluation of self-healing properties of the epoxy resin with disulfide links in its structure according to the invention.

Example 1 :

To quantify the healing efficiency of the self-healing crosslinked polymer according to example 1 , the recovery of the strength was determined using a tensile tester. In figure 7 a schematic diagram of the performed experiment is shown. First, the strength of the virgin material according to figure 1 was measured. In the next step, the samples are broken or cut. Afterwards the broken parts were put into contact. Then, it was allowed to heal at 60 ⁰ C for some time, and finally the strength was measured again.

Tensile tests were performed on dumbbell-shaped tensile bars (20 x 5 x 2 mm) at 0.5 mm/s using a 20 N force cell on a Zwick Z010 tensile tester. The equipment was controlled with TestXpert v11.02 software. Healing experiments were performed at 60 ⁰ C by bringing cut or broken samples together.

Representative stress-strain curves for the original material according to figure 1 are plotted in figure 8, figure 8A, showing that the elongation-at-break is close to 70% with a good reproducibility. When a sample fractures during the tensile test and the fracture surfaces are immediately put into as close as possible contact and heated at 60 ⁰ C, the mechanical properties are fully restored in just one hour (see figure 8B). As expected, longer healing times lead to better healing, but even when the contact time between the two broken parts is as short as 15 minutes, a repaired sample shows an elongation-at-break close to 40%. Surprisingly, for all the healing times, stress-strain curves superimpose and show only different elongations-at-break, which indicates that the healed samples have similar elastic properties as the original material. This material can be healed efficiently multiple times, and the mechanical properties after the second and third healing process are, within experimental error, fully restored (see figure 8C).

The healing efficiency was also evaluated optically. Figure 9 shows optical microscope images before (figure 9A) and after (figure 9B) the healing process. Figure 9B shows that the cut heals almost completely to produce a homogeneous material. A minor defect can be seen in the left side, which is most likely due to a slight offset when putting together the surfaces.

Synthesis of a cross-linked epoxy resin with different concentration of disulfide links in its structure.

The network was formed from a stoichiometric mixture of two epoxy resins, one free of disulfide groups (DER732, Dow Chemicals, epoxy equivalent = 303 g/eq) and one another containing disulfide groups in its structure (EPS25), and Tetrathiol. Different compositions were prepared containing different disulfide concentrations. The compositions of the mixtures are listed in Table 1 , whereby samples 1 to 4 are samples according to the invention and sample 5 is a comparative sample.

Sample EPS25 DER732 Tetrathiol S-S

(mmol) (mmol) (mmol) (wt%)

1 3.44 3.82 3.63 20

2 2.58 5.97 4.27 15

3 1.72 8.08 4.9 10

4 0.86 10.23 5.5 5

5 0 4.95 2.47 0

To quantify the healing efficiency, recovery of the strength of the healed material was determined using a tensile tester in a similar way as already explained. Healing experiments were performed at 80 ⁰ C for 2 hours by bringing cut samples together.

The recovery of strength of healed samples with different concentrations of disulfide groups (20, 15, 10, 5 and 0 % wt) are shown in figure 10. Column 1 is equal to sample 1 , column 2 is equal to sample 2, column 3 is equal to sample 3, column 4 is equal to sample 4 and column 5 is equal to sample 5. The self-healing efficiency depends on the concentration of the disulfide groups present in the material. The material with the highest disulfide concentration shows the best self- healing efficiency, whereas the material without disulfide groups 5 does not show any self-healing property at all.

Example 2

To quantify the healing efficiency, the recovery of the strength of the healed material according to example 2 was determined using a tensile tester in a similar way as explained in Example 1. Healing experiments were performed at 140 ⁰ C for one (curve 15), two (curve 14), three (curve 13) and five (curve 12) hours by bringing cut samples together. The stress-strain curves for the virgin material (curve 11 ) and at different healing times are plotted in figure 12A. The virgin material (curve 11 ) has an elongation-at-break close to 80%. As expected, longer healing times result in a better recovery of the mechanical properties and after 5 hours (curve 12) the mechanical properties are restored. In figure 12B the healing efficiency as function of the healing time calculated for 5 samples is shown, whereby block 12 corresponds to the self-healing material after 5 hours healing time, block 13 corresponds to the self-healing material after three hours healing time, block 14 corresponds to the self-healing material after two hours healing time and block 15 corresponds to the self-healing material after one hour healing time.

As is shown in figure 12C, the recovery of the mechanical properties as a function of temperature (140 ⁰ C (curve 17), 120 ⁰ C (curve 18) and 100 ⁰ C (curve 19)) was determined after five hours of healing by bringing cut samples together. Curve 16 represents the virgin material according to example 2. Figure 12 C shows, that with higher temperatures, more recovery of the mechanical properties is achieved.